1. Field of the Invention
The present invention relates to a wide band frequency generating apparatus used as a local oscillator in a wireless receiver, such as a wireless modem, a wireless phone, a TV receiver, and more particularly, to a wide band frequency generating apparatus for expanding a tuning range using a frequency dividing method of dividing an oscillating frequency of an oscillator.
2. Description of the Related Art
Generally, an oscillator (OSC) is known as an apparatus for generating alternative current signal having a predetermined frequency and has been used in a wireless receiving apparatus for transmitting and receiving a signal through a predetermined channel, such as a wireless phone, a wireless modem, and a television receiver.
FIG. 1 is a conventional direct conversion type tuner in a wireless receiver. As shown in FIG. 1, the direct conversion type tuner includes mixers 3, 4 mixing a predetermined frequency input signal RFin having a predetermined level adjusted by a pre-amp 1 and an automatic gain controller (AGC) 2 with corresponding ones of oscillation signals Fo1, Fo2 generated from a local oscillator 5 to I and Q signals of a base band. The local oscillator 5 outputs the oscillation signals Fo1, Fo2 having the same frequency and a different phase angle, e.g., a phase difference of 90 degrees, to the mixers 3, 4. The local oscillator 5 may include an oscillator generating a frequency signal having the same frequency as a carrier of a selected channel, and a signal divider dividing the frequency signal into the oscillation signals Fo1, Fo2 having the different phase angle, e.g., the phase difference of 90 degrees.
The mixers 3,4 mix the frequency input signal RFin with corresponding ones of the oscillation signals Fo1, Fo2 to output sum signals and difference signals, respectively. The sum signals are not used since a frequency of the sum signals of the mixers 3, 4 is too high to use in a tuning system. Accordingly, the sum signals of the mixers 3, 4 are ignored by a characteristic of a circuit and parts of the tuning system, and the difference signals of the mixers 3, 4, e.g., the I, Q signals, are output.
Low pass filters (LPF) 6, 7 remove signals other than base band signals corresponding to the I, Q signals from the difference signals output from the mixers 3, 4. When the local oscillator 5 generates the oscillation signals Fo1, Fo2 having the same frequency as the carrier of the selected channel, the frequency input signal RFin having the same frequency as the carrier of the selected channel is mixed with the oscillation signals Fo1, Fo2 to generate the I and Q signals of the base band.
Therefore, the local oscillator 5 should be able to generate the frequency signal corresponding to a frequency receiving (tuning) band (range) of the tuner system. That is, the frequency receiving (tuning) band (range) of the tuner system should be the same as a frequency receiving band of a wireless system.
For example, a broadcasting band used in a satellite system is a range of about 950 MHz˜2.15 GHz. Accordingly, the oscillator 5 used in a tuner of a satellite broadcasting receiver should be able to generate a frequency oscillation signal of a band of about 950 MHz˜2.15 GHz.
The oscillator 5 should have the frequency tuning range of 125% of the receiving band of the tuner according to a frequency tuning range calculated by a maximum-minimum ratio expression method of expressing an efficiency of the oscillator 5 as follows.
                    f        max            -              f        min                    f      min        ×      100    ⁡          [      %      ]      
However, the frequency tuning range is about 30% in a voltage control oscillator (VCO) including an internal oscillator in an integrated circuit. In order to expand the frequency tuning range, various developing research activities have been carried out.
As shown in FIG. 2, the VCO generally includes an active circuit 21 outputting an oscillation frequency signal in response to a frequency signal generated from an oscillation circuit 22 which has an inductance and a capacitance to determine a frequency of the frequency signal.
FIG. 3 is a circuit of an active circuit unit of a conventional VCO shown in FIG. 2. In response to an input voltage controlled by control signals A, B, a capacitance value of varactor diodes VD1, VD2 varies. The oscillation frequency signal f is obtained from an inductance L and a capacitance C of varactor diodes VD1, VD2 according to a control voltage Vctrl, which is controlled by control signals A and B by using a formula:
  f  =            1              2        ⁢                                  ⁢        π        ⁢                  LC                      .  
In the conventional VCO, since a variable range of the capacitance C of the varactor diodes VD1, VD2 is a major factor in determining the frequency tuning range, the various developing research activities expanding a capacitance variable range of the varactor diodes VD1, VD2 as a method of expanding the frequency tuning range of the oscillator, are performed.
However the expanded frequency tuning range is not able to satisfy industrial demands required in the tuner of the satellite-broadcasting receiver since the expanded frequency tuning range through a development of the varactor diodes is about 40%. Therefore, a plurality of oscillators or another methods of changing the oscillation circuit may be used in the tuner of the satellite-broadcasting receiver.
FIG. 4 is a circuit of another active circuit unit modified from the active circuit unit of the conventional VCO shown in FIG. 2. The another active circuit unit includes a plurality of capacitors C1 through C6 connected in parallel to the varactor diodes VD1, VD2 which are selectively connected to the capacitors C1 through C6 by on and off of a plurality of switch array sw1 through sw6 to expand a total capacitance value Cvar of the oscillation circuit unit 22.
The total capacitance value Cvar of the oscillation circuit unit 22 as shown in FIG. 4, varies according to control signals Vsw1, Vsw2, Vsw3 of the switch array sw1 through sw6. By selecting the capacitors C1 through C6, a center frequency of the oscillation circuit unit 22 varies step by step, and a frequency range between the steps is covered by a variable range of the capacitance of the varactor diodes VD1, VD2 to expand a total frequency variable range of the oscillation circuit unit 22.
In this case, the frequency tuning range may be expanded to 70%˜80%. However, at least two oscillators must be used to satisfy the frequency tuning range of 150% which is required in the tuner of the satellite broadcasting receiver as describe above.
Therefore, as shown in FIG. 5, two oscillators are used to satisfy the frequency tuning range of the tuner of the satellite-broadcasting receiver.
Moreover, the active circuit unit of FIG. 4 is added to the oscillation circuit unit installed in an inside of the VCO, a load on the active circuit unit increases, and a current consumption also increases. Furthermore, since a resistance characteristic of the switch array sw1˜sw2 provided in the oscillation circuit unit 21 decreases a quality factor of the capacitors, a phase noise of the VCO increases.